The sudden death of Antonin Scalia and ensuing fight over the process to replace him on the Supreme Court has created a vexing election-year problem for Senate Republicans, who – a mere nine months before November – are now caught between the competing demands of their conservative allies and moderate voters who could make-or-break the party’s already imperiled majority.

In what might amount to their most high-profile decision of their campaigns, vulnerable Republican incumbents can side either with ideological allies who believe viscerally important issues like abortion-rights, immigration reform, and government overreach are at stake – or with moderates who are more broadly interested in lawmakers who lessen government dysfunction and help get things done.

The pressure is especially intense for a quintet of swing-state Republican incumbents who have a direct say on if and how the Senate considers President Barack Obama’s appointee: Mark S. Kirk of Illinois, Ron Johnson of Wisconsin, Kelly Ayotte of New Hampshire, Patrick J. Toomey of Pennsylvania, and Rob Portman of Ohio. Indeed, for a collection of senators whose biggest headache to date has been one outside of their control – Donald Trump’s improbable success in the GOP presidential primary – the question of how to handle Supreme Court confirmation is at least within their power, but it could be no less challenging.

Confident that they were seizing a unique political opportunity, Democrats were already pressuring Republican incumbents on Sunday to break from their conference chief, Senate Majority Leader Mitch McConnell, who said Saturday that the Senate shouldn't confirm a new Supreme Court justice before the next president takes office. The Democrats’ message: Either defy your party’s leadership or face the consequences on Election Day.

“The Constitution is clear: the president is empowered to nominate Supreme Court justices,” said Lauren Passalacqua, spokeswoman for the Democratic Senatorial Campaign Committee. “This vacancy may not be convenient for a Republican majority that has already abandoned any intention of governing this election year, but they have a responsibility to our nation. If Mitch McConnell and the GOP caucus follow through on their alarming plan to obstruct this constitutional process, then voters will ensure they will not keep the majority for long.”

The electoral fates of Portman, Toomey, Ayotte, Johnson, and Kirk – along with open-seat races in Florida and Nevada and the re-election campaign of Democratic Sen. Michael Bennet – will determine which party controls the Senate majority in 2017. If the party’s presidential nominee wins, Democrats need to win a net of four seats to regain control of the chamber; the number grows to five if the GOP presidential nominee wins.

The five incumbent Republicans are under particular pressure because Obama won their home states during both of his presidential campaigns, and each figures to be a top target of the next Democratic presidential nominee in the fall. To win re-election there, all of the GOP lawmakers must reach beyond their conservative base to assemble a winning coalition.

It’s little surprise that by Sunday night, only two of the Republican senators, Ayotte and Johnson, had weighed in on the confirmation process. Both urged the Senate to hold off confirming a new justice until next year.

“We’re in the midst of a consequential presidential election year, and Americans deserve an opportunity to weigh in given the significant implications this nomination could have for the Supreme Court and our country for decades to come,” Ayotte said in a statement. “I believe the Senate should not move forward with the confirmation process until the American people have spoken by electing a new president.”

Ayotte has faced pressure on both her right and left flank since her re-election effort began. Conservatives have threatened to launch a primary campaign against her because of moderate stances she’s taken on the environment and Export-Import Bank, along with her decision to involve herself in a local state House leadership election.

Her decision was blasted by her Democratic opponent, New Hampshire Gov. Maggie Hassan, who called it a “complete abdication of the Senate’s constitutional duty.”

“Senator Ayotte's decision to put her party leaders ahead of our country is a sad reflection of just how wrong her priorities are and how broken Washington has become,” Hassan said.

While Kirk, Toomey and Portman expressed their respect for Scalia and condolences to his family, they didn’t clarify a position on the process. The National Republican Senatorial Committee, the political arm of the Senate GOP, also declined to comment.